Crossover temperature spectroscopy

Independent of whether or not a well-defined crossover temperature can be observed in NS data above Tg, it has been well known for a considerable time that on heating a glass from low temperatures a strong decrease of the Debye-Waller factor, respectively Mossbauer-Lamb factor, is observed close to Tg [360,361], and more recent studies have confirmed this observation [147,148,233]. Thus, in addition to contributions from harmonic dynamics, an anomalously strong delocalization of the molecules sets in around Tg due to some very fast precursor of the a-process and increases the mean square displacement. Regarding the free volume as probed by positron annihilation lifetime spectroscopy (PALS), for example, qualitatively similar results were reported [362-364]. 

When water is confined in a hydrophobic substrate, it exhibits a lower 7p than water confined in a hydrophilic substrate, and the protein hydration water crossover temperature decreases with pressure. This P effect reflects the increase in the protein-water interaction and the increase in the water s ability to access the protein hydrophobic core [32]. Using UV spectroscopy, similar P effects on the dynamic properties of biomolecules have been studied in p iactoglobulin [33], which is also a sensitive food protein. These results suggest that these pressure effects on proteins have universality. 

The driving force for the temperature-dependent spin crossover (SCO) is the entropy difference between the HS and the LS isomers which arises mainly from a shift of the vibrational frequencies when passing from the HS to the LS state [97-99]. This frequency shift has been studied by IR- and Raman-spectroscopy and recently also by NIS [23, 39, 87]. The NIS method is isotope ( Fe) selective and, therefore, its focus is on iron-ligand bond-stretching vibrations which exhibit the most prominent contribution to the frequency shift upon SCO [87]. 

About twenty years ago we reported on the di-isothiocyanato iron(II) complex of the tetradentate ligand tpa (tris(2-pyridylmethyl)amine) [7] (6). It was shown that this complex exhibits the spin crossover phenomenon with a critical temperature Tm of about 170 K. Several different solvated phases of the same system have since been characterized by Chansou et al. [8]. The unsolvated phase which can be isolated from an aqueous solution has been investigated by nuclear forward scattering (NFS), nuclear inelastic scattering (NIS) [9], extended x-ray absorption fine structure (EXAFS) spectroscopy, conventional Mossbauer spectroscopy, and by measurements of the magnetic susceptibility (SQUID) [10-13]. The various measurements consistently show that the transition is complete and abrupt and it exhibits a hysteresis loop between 102 and 110 K. 

The special feature of the spin crossover process in all bpym-bridged dinuclear compounds studied so far is the occurrence of a plateau in the spin transition curve. A reasonable assumption to account for this observation is that a thermal spin transition takes place successively in the two metal centres. However, it cannot be excluded that spin transition takes place simultaneously in the dinuclear units leading directly from [HS—HS] pairs to [LS-LS] pairs with decreasing temperature. Therefore, two possible conversion pathways for [HS—HS] pairs with decreasing temperature may be proposed [HS—HS]<->[HS—LS]<->[LS—LS] or [HS-HS] [LS-LS]. The differentiation of the existence of the [LS—LS], [HS—LS], and [HS—HS] spin pairs is not trivial and has recently been solved experimentally by utilisation of magnetisation versus magnetic field measurements as a macroscopic tool [9], and by Mossbauer spectroscopy in an applied magnetic field as a microscopic tool [11]. 

Spin-crossover phase transition of a manganese(IU) complex [Mn(taa)] was studied by variable-temperature laser Raman spectroscopy and it was found that the vibrational contribution in the transition entropy is not dominant in contrast to the cases of ordinary iron spin-crossover systems. The discovery of a dynamic disorder in the HS phase by means of dielectric measurements provided an alternative entropy source to explain the thermally induced spin-crossover transition. This dynamic disorder was attributed to the reorienting distortion dipoles accompanying the E e Jahn-Teller effect in HS manganese(III) ions. 

A systematic study of the metal dilution effect on the spin-crossover behaviour in the solid solutions [FexZnj x(2-pic)3 ] Cl2 EtOH (0.0009 < x < 1) has been performed employing S7Fe Mossbauer spectroscopy between 5 and 300 K 84.8S). The purpose was to find support for the cooperative domain model suggested earlier by Sorai and Seki34 87). Some representative Mossbauer spectra of the undiluted system (x = 1) as a function of temperature are displayed in Fig. 19. The spectra demonstrate that... 

The Lamb-Mossbauer factor /lm, which is difficult to determine experimentally as well as compntationally, does not belong to the most common Mdssbaner parameters in the field of inorganic chemistry. It can be of importance, however, when Mossbauer spectra are used to determine for a given sample the fractions of different species containing the same Mossbauer isotope. This is the case, for instance, for iron spin-crossover complexes where Mdssbaner spectroscopy can be used to measure the temperature- or pressure-dependent fraction of different spin isomers. The first approach to estimate the change of the Lamb-Mdssbaner factor upon spin crossover has been restricted to molecular vibrations, neglecting the important contribution that arises from intermolecular vibrations. ... 

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